The invention relates generally to the field of radar and radio receivers and in particular to an interference suppressor and a Global Positioning System (GPS) navigation system receiver employing such an interference suppressor.
The Global Positioning System (GPS) is a navigation system based on eighteen satellites in orbit. When fully operational the eighteen satellites will be evenly dispersed in three, inclined, 12-hour circular orbits chosen to ensure continuous 24-hour coverage. The GPS provides extremely accurate time and three-dimensional position and velocity information to users anywhere in the world. Normally, four satellites are required for precise location determination in four dimensions (latitude, longitude, altitude and time). The location determinations are based on measurement of the transit time of RF signals from the satellites selected from the total of eighteen. Each satellite transmits a different pair of L-band carrier signals including an L1 signal at 1575.42 MHz and an L2 signal at 1226.6 MHz. The L1 and L2 signals are biphase modulated by two pseudo-random noise (PN) codes comprising a P-code which provides for precision measurement of transit time and a C/A (course/acquisition) code which provides for a course measurement of transit time and provides for easy lock-on to the desired signal suitable for many commercial purposes. Since each satellite uses different PN-code sequences, a signal transmitted by a particular satellite can be selected by generating and matching (correlating) the corresponding PN-code pattern.
Phase code modulation is ideally suited to measuring time of time delay. The time delay is a measure of range while the difference in phase measurements taken at fixed time intervals is a measure of frequency. The phase code modulation is compared (correlated) with the expected phase or replica of the phase code modulation. Typical phase code modulations include not only Binary Phase-Shift Keying (BPSK), but also Binary Frequency-Shift Keying (BFSK).
The P-code is the principal navigation pseudo-random noise (PN) ranging code of the Global Positioning System. The P-code is a repetitive sequence of bits referred to as chips (in spread spectrum parlance). The P-code for each satellite is the product of two PN-codes X1 and X2 where X1 has a period of 1.5 sec or 15,345,000 chips and X2 has a period of 15,345,037 or 37 chips longer. The P-code generator in a GPS receiver reproduces a replica of the P-code that is generated by a P-code generator of a particular GPS satellite and each satellite produces a unique P-code. The C/A code is a relatively short code of 1023 bits or 1 msec duration at a 1.023 Mbps bit rate. This code is selected to provide good multiple access properties for its period.
For both military and commercial applications the ability of the GPS receiver to resist jamming or interference is very important. With the typical spread spectrum signal, the received signal is below the receiver noise level. Examples for the typical spread spectrum pseudo-noise (PN) code rates are:
______________________________________ PN-CODE PN-CODE PN-CODE TRACK DATA DEMODULATION R.sub.c I/S = 10 LOG R.sub.c I/S = LOG(R.sub.c /2R.sub.d) ______________________________________ 1/2 MHz 57 dB 37 dB 1 MHz 60 dB 40 dB 5 MHz 67 dB 47 dB 10 MHz 70 dB 50 dB ______________________________________
where: R.sub.c =the PN-code chip rate, and the data modulation rate (R.sub.d) equals 50 Hz. When data is modulated on the PN-code bit stream, it is also assumed to be a random stream of data bits. The signal-to-interference power ratio is discussed in a paper entitled "GPS Signal Structure and Performance Characteristics," by J. J. Spilker, Jr., reprinted by The Institute of Navigation, Global Positioning Papers, published in Navigation, Vol. 2, pp. 29-54. Spilker shows on page 42 that the signal-to-interference ratio equals R.sub.c /2R.sub.d ; with 50 Hz data modulation the signal-to-interference power ratio for the P-code is 10 log (R.sub.c /2R.sub.d) or 50 dB. Hence, GPS receivers are known to provide 50 dB of interference discrimination; however, in higher interference environments, such as 100 dB interference-to-signal (I/S) ratio, a GPS receiver does not operate properly. Adaptive filters have been employed to put notches in the frequency bandwidth and provide approximately 25 dB interference discrimination thereby providing a total of 75 dB of total interference discrimination. RF amplitude suppression using a null steering antenna is a technique to provide approximately 25 dB additional interference discrimination. Cascading such a null steering antenna, an adaptive filter and a GPS receiver can provide 100 dB of interference discrimination. However, the cost and size constraints of such prior art technologies have made this level of interference discrimination prohibitive for certain applications. There exists a need to employ GPS for guidance and navigation applications having an I/S ratio of equal to or greater than 100 dB, but such applications require a low cost, small size, light weight and low power GPS system. In order to meet these requirements an alternative means to the use of adaptive filters is described herein which also provides 25 dB interference discrimination.